You can book “Stand S Seats” here:
General sale (all ticket categories): from Feb 1st, 10am JST on Japanese playguides
Ticket shopping services (e.g. Tickets Galore Japan) can help you purchase from Japanese ticket agencies or Japanese resellers.
Want to know everything about buying concert tickets in Japan? Start with The 5 Basics About the Japanese Ticketing System
*Disclaimer: Information and currency exchange rates as of January 21st, 2020 but subject to change without notice / accuracy of this information is not guaranteed / make sure to double-check if you are planning to attend these shows